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Voters divided ahead of Pennsylvania election

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Voters divided ahead of Pennsylvania election02:31

(CNN)Even a victory in Pennsylvania's 18th House District on Tuesday could be bad news for national Republicans.

For starters, the seat has no bearing on the balance on power in the House as Republicans hold a clear majority. And after a court mandate that new maps go into effect, this seat will likely be eliminated next Congress.

So the real importance of Tuesday's special election pitting Republican Rick Saccone against Democrat Conor Lamb is, instead, what the results say about the strength of each party heading into November's midterm elections.

Republicans may hope that a win will provide them with an ego boost. But history tells us it's far more significant to look at the margin between Lamb and Saccone, not at who ultimately wins or loses. And if we're only looking at the margin, it's pretty clear that the result in Pennsylvania's 18th could very likely end up being bad news for Republicans.

We know from past years that the average special election can tell us a lot about what will occur nationally in the midterms, so long as we look at how much the margins in them deviate from past margins in the district.

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Former GOP Rep. Tim Murphy, who ran unopposed in both 2016 and 2014, won the district by 28 points in 2012. Republican President Donald Trump won it by about 20 percentage points, when he was losing the national popular vote by 2 points. It's a district that Republican Mitt Romney won by 17 points, when he was losing the national popular vote by 4 points.

In other words, the race shouldn't be close even if the national environment was neutral. Saccone should be winning by double digits.

Yet, the available polls show Lamb and Saccone neck-and-neck. An average of thethreesurveys, none of which meet CNN's polling standards, taken within three weeks of the election actually have Lamb up by a point. That means this race is a tossup given the true margin of error with 95% confidence for special House election polling averages (in which there is at least two polls part of the average) being about +/- 10 percentage points.

Now, any single special election can be a false positive (or false negative, depending on your angle). Eight years ago, Democrats won a closely watched special election in southwest Pennsylvania, but Republicans romped in the 2010 midterms. This year, most observers regard Lamb as a strong candidate, while Saccone is seen as a weak fundraiser. Additionally, the district is ancestrally Democratic, as seen through the 6-point Democratic advantage in party registration in the district. Party registration in the past has been crucial in explaining House results, even after controlling for the presidential lean of the district.

Some of the Democratic edge may reflect people not changing their registration to align with their current feelings towards federal politics. It could also reflect some good will left towards Democrats in the district, as demonstrated by some Democratic strength in local elections.

But there's ample reason to think that Pennsylvania 18 is more of an indicator of the results in November than not. A close race there would be right in line with what other special elections have been showing so far. In the average federal special election this cycle, Democrats have been outperforming their baseline based off the prior two presidential elections by 16 percentage points.

A 16-point swing in federal elections lines up with a very good year for the Democrats in the House vote come the fall.

To match that 16 point average, Lamb needs only to come within 5 points of Saccone on Tuesday.

A close Pennsylvania 18 result would also be another datapoint to demonstrate that Republicans are losing the most ground in some of Trump's strongest areas. We know that some of the Republicans' biggest under-performances in special elections relative to the presidential baseline have been in states that flipped from Barack Obama to Trump (such as Iowa, Pennsylvania and Wisconsin) and states along the Appalachian Mountains where coal had, at least at one point, played a significant part of the economy (such as Kentucky, Pennsylvania and Tennessee).

These are areas where Democrats may try to be more competitive in the 2018 midterms.